Do you experience problems during downloading?
Check your connection in Status and Interface settings window.
Use Test Server in Config > Servers.
We will probably ask you to do a test using only basic settings.

Do you experience problems during repair or unpacking?
Enable +Debug logging in the Status and Interface settings window and share the relevant parts of the log here using [ code ] sections.

Since I get an error 500 using the search function on the forum, I do not know if this has been discussed or not: when adding a server, SabNZB requests a password. When I fill it in, it is in clear text. Would it be possible to have a checkbox next to it, to "censor" the content I fill in? I usually use SabNZB to introduce new people to Usenet and I do not want them to know my password.

I think that it probably has been done so, to prevent Chrome / Firefox to remember the passwords.

Yes indeed, it's top stop Chrome/FF/LastPass from storing the password.
We had many many many many users where the browser auto-filled usenet-passwords as their SABnzbd-main password.
You can request this feature on GitHub: https://github.com/sabnzbd/sabnzbd/issues